1. Technical Field
This invention relates to the field of fluid sensors. More specifically the invention relates to fluid abrasion sensors, fluid corrosion sensors, combined abrasion and corrosion sensors, and a method for sensing abrasion and/or corrosion.
2. Background Art
Abrasion sensors and corrosion sensors are particularly important when an increase in the abrasiveness or corrosiveness of a fluid, such as oil, aqueous solution, or water, may damage equipment or upset a process. Some sensors exist that monitor the abrasiveness or corrosiveness of a fluid and yield a quantitative value such as milligrams of particulate matter per liter of fluid for abrasiveness or the pH for corrosiveness. Such sensors are typically expensive to purchase, use, and maintain and often may be used only in limited circumstances due to their fragility. Also, a quantitative value is often not needed so long as a sensor indicates when the abrasiveness or corrosiveness of a fluid exceeds an allowable limit. This is particularly true for internal combustion engines, hydraulic systems, and the like wherein abrasive particles and/or corrosive acids readily accumulate in oil circulating through such a device or system.
One type of sensor exists that includes a sacrificial amount of a conductive substance. The conductive substance is selected so that it will abrade or corrode when the respective abrasiveness or corrosiveness of the subject fluid exceeds an allowable limit. Also, the conductive substance is selected to corrode or abrade rapidly enough that the condition may be detected before significant damage to equipment or processes occur. Essentially, such a sensor will exhibit a given resistance (typically measured in ohms) when newly exposed to the subject fluid, but the resistance will increase as the conductive substance wears away, typically by abrasion or corrosion. Most commonly, such sensors are connected to electronic circuitry capable of measuring the resistance of the sensor. The electronic circuitry is used to apply a voltage across the sensor and measure the resulting current. The equation R=V/i, wherein R is resistance, V is voltage and i is current, can then be used to calculate the measured resistance. For abrasion sensors, such devices are described in U.S. Pat. No. 4,305,278 to Stewart et al., U.S. Pat. No. 4,337,668 to Zupanick, and U.S. Pat. No. 5,211,677 to Sargeant et al., each of which is hereby incorporated by reference for their pertinent and supportive teachings. For corrosion sensors, such devices are described in U.S. Pat. No. 3,108,242 to Scott, Jr., U.S. Pat. No. 3,124,771 to Rohrback, U.S. Pat. No. 3,854,087 to Frenck et al., U.S. Pat. No. 3,857,094 to Caldecourt, U.S. Pat. No. 4,217,544 to Schmidt, U.S. Pat. No. 4,326,164 to Victor, U.S. Pat. No. 4,338,563 to Rhoades et al., U.S. Pat. No. 4,741,204 to Luck et al., U.S. Pat. No. 4,780,664 to Ansuini et al., U.S. Pat. No. 5,243,297 to Perkins et al., and U.S. Pat. No. 5,332,961 to Hammerle, each of which is hereby incorporated by reference for their pertinent and supportive teachings.
Many types of fluids, for example, lubricating oil and hydraulic fluid, contain additives to prevent the buildup of corrosive compounds. Corrosion typically is limited if the fluid contains the proper amount and type of additives. The corrosive compounds are still generated, but the additives change the chemical characteristics of the corrosive compounds such that corrosion is suppressed. However, as the fluid ages, the additives are depleted and corrosive compounds begin to accumulate in the fluid. When a corrosion sensor of the type described above is present, the conductive substance will begin to corrode and the sensor's resistance will change, thus signaling an accumulation of corrosive compounds. Typically, an elemental metal such as lead or copper is selected for the conductive substance. It is a problem among the metals most often used that their resistivity is temperature dependent and a change in temperature of the fluid may be mistaken for an increase in corrosiveness. Accordingly, conventional corrosion sensors include a temperature reference for performing temperature correction of any changes in the measured resistance. Often, a Wheatstone bridge or Kelvin bridge arrangement is used as shown in several of the references indicated above. Unfortunately, requiring a temperature reference adds to the complexity and cost of a corrosion sensor and a sensor that does not require temperature correction is highly desirable.
It is also a problem among the metals most often used that their resistivity is relatively low, typically yielding corrosion sensors having a resistance of 1 ohm or less. It is a significant disadvantage of conventional corrosion sensors that their low resistance necessitates a relatively low applied voltage, allowing electrical noise to significantly influence measuring the resistance. This condition renders it difficult at times to reliably determine whether a change in resistance is from corrosion or just electrical noise. Accordingly, a sensor that reliably distinguishes between noise and corrosion is highly desirable.
Many types of equipment and processes that use fluids, for example, engines and hydraulic systems, include devices, for example, filters, to remove abrasive matter and prevent its buildup in the fluid. Abrasion typically is limited if the device works properly, however, the device may rupture or clog and allow abrasive matter to pass through it or around it via a bypass valve. When an abrasion sensor of the type described above is present, the conductive substance will begin to abrade and the sensor's resistance will change, thus signaling an accumulation of abrasive matter. Typically, an elemental metal or metal alloy such as nickel-chromium is selected for the conductive substance. It is a problem of many metals that their resistivity is temperature dependent as discussed above for corrosion sensors and a temperature reference is required. Unfortunately, requiring a temperature reference adds to the complexity and cost of an abrasion sensor too, so a sensor that does not require temperature correction is highly desirable. Also, as discussed above for corrosion sensors, it is a problem of conventional abrasion sensors that they have a resistance of 1 ohm or less. Accordingly, an abrasion sensor with a greater resistance to reliably distinguish between noise and abrasion is highly desirable.
Thus, it can be seen from the above discussion that it would be an improvement in the art to provide an abrasion sensor and a corrosion sensor that do not require temperature correction and reliably distinguish noise from abrasion or corrosion.